ECHO PATH DELAY SIMULATOR FOR USE WITH ADAPTIVE ECHO CANCELLERS

• US 3,721,777 A
• Filed: 11/26/1971
• Issued: 03/20/1973
• Est. Priority Date: 11/26/1971
• Status: Expired due to Term

• Current Assignee: Edmond Joseph Thomas
• Original Assignee: Edmond Joseph Thomas
• Inventors: Thomas, Edmond Joseph
• Primary Examiner(s): Claffy, Kathleen H.
• Assistant Examiner(s): Faber, Alan
• Application Number:
  US05/202,338
• Time in Patent Office: 480 Days
• Field of Search: 179/170.2,170.8
• US Class Current: 379/406.08
• CPC Class Codes: H04B 3/23 using a replica of transmitted signal in the time domain, e.g. echo cancellers

Abstract

The echo path of a four-to-two wire junction is bridged by a variable delay device and an adaptive echo canceller connected in series. If the variable delay device is controlled so as to provide a delay to an incoming signal equal to the echo path delay, the number of taps required in the echo canceller can be reduced and the stability of the canceller thereby improved. To this end, a delay measuring circuit serves to form the crosscorrelation function between the incoming or echo generating signal and the echo. The point at which this cross-correlation function is first determined to be a maximum corresponds to the echo path delay. The determination is then used to control the variable delay device so as to achieve a delay therein substantially equal to the echo path delay.

Claims

1. Echo cancelling apparatus comprising a variable delay device and an adaptive echo canceller connected in series across the echo path of a four-to-two wire junction, means bridging the echo path and serving to form the cross-correlation function between an incoming signal to the same and its echo, means for determining the point at which the cross-correlation function is first a maximum, and means operative in response to the determination of the last-recited means to control the variable delay device to achieve a delay therein substantially equal to the echo path delay.

2. In echo cancellation apparatus which includes a variable delay device and an adaptive echo canceller connected in series across the echo path of a four-to-two wire junction, said cancellation apparatus being characterized by means bridging the echo path and serving to form the cross-correlation function between an incoming signal to the echo path and its echo, means for determining the point at which the cross-correlation function is first a maximum, and means coupled to the determining means for controlling the variable delay device to achieve a delay therein substantially equal to the echo path delay.

3. In echo cancellation apparatus which includes a delay device and an adaptive echo canceller series coupled across the echo path of a four-to-two wire junction, said cancellation apparatus being characterized by means for measuring the delay between an input signal to the echo path and the return echo signal and producing a signal for controlling the delay of said delay device in accordance therewith, the delay measuring means comprising a tapped delay line, means coupling said input signal to the tapped delay line, said delay line having a plurality of output taps with a delay between successive taps of T seconds, means for respectively multiplying the signals at the respective delay line taps with the echo signal, means for respectively integrating the product signals of the multiplying means over a predetermined period of time, and comparison means for determining the maximum of the integrated product signals, said maximum corresponding to the delay in said echo path.

4. Echo cancellation apparatus as defined in claim 3 including means operative in response to the aforementioned maximum determination to control said delay device so as to provide a delay therein substantially equal to the eCho path delay.

5. Echo cancellation apparatus as defined in claim 4 wherein the tap delay T is equal to the Nyquist interval.

6. Echo cancellation apparatus as defined in claim 5 wherein said delay measuring means is time-shared between a plurality of echo cancellers.

7. A method for measuring the delay between the input signal x(t) to an echo path and the return echo signal y(t) comprising the steps of delaying the input signal in increments of T so as to produce the delayed replicas x(t-T), x(t-2T)...x(t-NT), multiplying each of the replicas with the echo y(t), integrating the respective product signals of the respective multiplications over a predetermined period of time, and determining the maximum of the integrated product signals, said maximum corresponding to the delay of said echo path.

8. The method as defined in claim 7 wherein T is equal to the Nyquist interval of 1/2B where B is the bandwidth of the echo path in Hertz.